The present invention concerns an improvement for a developing roller of monocomponent developing system used in image forming apparatus such as copying machines, printers and facsimile units.
Heretofor, as a suitable developing system for realizing an image forming apparatus small in size and light in weight, there has been known a monocomponent developing system which does not use carriers usually applied in the two component developing system.
A developing roller of the monocomponent system comprises a core metal (conductive supporting body) and a dielectric layer formed at an outer surface of the core metal, and as an example of the layer, a resin layer can be exemplified.
Further, there is a roller of the monocomponent developing system having a three layer, namely, a core metal, a dielectric layer and a float electrode (conductive, about 100 .mu.m diameter), which is disposed on a surface of the layer to provide the same effect as an electrode effect caused by carriers in a two component developing system. The three-layer roller has an excellent grading property and simultaneously an excellent reproducibility for low contrast characters.
Among developing rollers of the monocomponent developing system, there is a roller which is given a magnetic force for a toner transportation and also for a reduction of toner layer thickness. These rollers can form a stable image of high quality by stabilizing their electric properties and limiting the properties in a certain range.
However, working circumstances of image forming apparatus have recently changed remarkably because a use of the apparatus being small and inexpensive are become very much popular recently. In spite of the above fact, there has been no sufficient consideration for a hygroscopic property (water absorption) of a resin forming a dielectric layer of conventional monocomponent developing roller. Accordingly, water absorption of the dielectric layer very much increases under high temperature and humidity as shown in FIG. 7 taking ethylene-propylenediene-methylene rubber (hereinafter referred to as "EPDM") as an example.
When the water absorption of the dielectric layer changes greatly depending upon its working circumstance, electric properties of the developing roller of monocomponent system, for instance, its volume resistivity, change greatly (see FIG. 8). As the result, amount of toner adhered to the roller or an electric charge of toner varies during development and transferred amount of toner from the roller to a light sensitive body fluctuates bringing problems of having irregularities development concentration and unstable image.
Practically, although there are various kinds of developing rollers having relatively susceptible volume resistivity and having relatively unsusceptible one to a change of water absorption of the rollers, in any way, as far as the change is big, most kinds of rollers are forced to change their resistivity and influence properties of the roller. When a material, such as polyamide (nylon), showing a big change of water absorption depending upon circumstantial conditions is used, a change of the volume resistibity becomes remarkable and such material can not be used in practical application. (Refer to FIGS. 9a and 9b).
Further, as to electric properties of the roller, especially a volume resistivity of the roller, which become necessary depending on conditions of a developing process, particularly properties of photosensitive body or toner, it is sometimes requested to be, for example, 10.sup.8 .about.10.sup.13 .OMEGA..cm, much lower than the resistivity of 10.sup.13 .about.10.sup.16 .OMEGA..cm which many resins generally have. In order to meet the requirement, barium ferrite, strontium ferrite or maganese-aluminum magnet is sometimes mixed with a resin. However, even in this case, if water absorption of the resin is high, its electric properties change similarly as above and become meaningless in practical use.
The present inventors have made an earnest study for dissolving the foregoing problems and, as a result, have found that these problems have effected by a water absorption in the dielectric layer of the developing roller of monocomponent developing system. Based on the finding they have accomplished the present invention.